Initially, Goose thought his problems occurred when his HR got above a certain level but it was actually caused by the visual patterns of the trees in the forest, not the exercise itself. Turns out he was able to ski in the middle of a frozen lake but not on a ski trail. One time he saw this photo on my AP log, started to feel ill and had to close his computer.
Every concussion is different, and our other friend's headaches became much worse when he tried some of the visual exercises that helped Goose.